Sheffield Scientists Crack the Secret of Rome's Color-Shifting Lycurgus Cup

By Grace Kim

Researchers at the University of Sheffield have recreated the color-changing effect of the British Museum's 1,600-year-old Lycurgus Cup, solving a mystery that has puzzled experts for centuries. The Roman vessel looks green in ordinary lighting but glows red when lit from behind.

"To me, it's magic," Lisa Hollands, a technician who worked on the project, told the Guardian. She noted that "people have tried it before with limited success" and described the undertaking as a "labor of love."

What is the Lycurgus Cup?

The cup is a Late Roman glass cage-cup (a vessel carved so the glass forms an openwork shell) that the organizing committee behind the UN-designated International Year of Glass named one of the seven glass wonders of the world in 2022. Its design draws on the sixth book of Homer's Iliad and celebrates the triumph of Dionysus, the god of wine, over a vine-wrapped King Lycurgus.

How does the color change work?

The Sheffield team emulated the cup's chameleonic quality by embedding gold and silver nanoparticles into glass. Each particle, as the Guardian put it, is "a thousand times smaller than a grain of sand." That scale is the whole trick: particles that small interact with light in ways ordinary glass doesn't.

Hollands explained the mechanism plainly. "So, for this glass, if light is shone through it, the gold and silver particles block blue and green light, only allowing reds and oranges to pass through," she said. "If light is reflected off the surface, the particles have the effect of scattering the light, giving the opaque green color we see."

In other words, one and the same object reads as two different colors depending on where the light source sits. That's a feat of materials control the Romans achieved without any modern theory of optics — which raises the obvious question.

What does this change for our understanding of Roman craft?

It confirms the effect wasn't accidental. Reproducing it required deliberate control of nanoparticle composition and dispersion, implying Roman glassmakers held reliable working knowledge of gold- and silver-doped glass (glass melted together with trace metals). Hollands joked about the gap between then and now: "If the Romans could have seen me experimenting, they'd probably laugh and ask: 'Why does she not know how to make this?'"

Ian Freestone, emeritus professor of archaeological materials at University College London, suggested a ritual reading of the green-to-red shift. "The depicted scene has grape vines pulling Lycurgus into the underworld," he said. "So it could have been used in a drinking ceremony of some sort." The color change could echo the ripening of a grape, from green to the red of wine.

Where to see the results

A related sculpture by glass artist Elliot Walker, commissioned by the University of Sheffield, will appear at the university's Festival of the Mind in November before entering the collection of its Turner Museum of Glass. It offers a rare chance to see the rediscovered technique in a contemporary object, alongside the ancient original that inspired it in London.